A zero$-$day attack occurs under which of the following conditions?

A$.$ When the malware's actions have been noticed and the vulnerability has been discovered

B$.$ When the malware's actions have been noticed and identified as an attack

C$.$ When the malware and the vulnerability have been identified but no fix is available yet

D$.$ When the malware has been written for a UNIX$-$based system and is found on a Windows computer